1. Field of the Invention
The invention relates initially to a heat-sensitive recording material comprising a web-shaped substrate and at least one heat-sensitive recording layer, formed on either or both of the sides of the web-shaped substrate. The heat-sensitive recording layer contains at least one dye precursor and at least one salicylic acid derivative as a (color) developer, color developer, or developer, reactive with this at least one dye precursor. Reactive with a dye precursor is to be understood as meaning that this at least one salicylic acid derivative responds to a sufficient supply of external heat by reacting with this at least one dye precursor to form a visually discernible printed image.
2. Description of the Prior Art
Recording materials that are heat sensitive and react color-formingly to the supply of external heat have been known for many years and enjoy a basically undiminished popularity, inter alia because their use is associated with immense advantages for businesses issuing tickets and/or sales receipts. Since the color-forming components, i.e., the dye precursors as well as the color developers that react therewith on heating—also known as (color) acceptors, acceptors, or color acceptors, reside in the recording material itself in such a heat-sensitive recording process, the thermal printers—which are consequently free from toner and ink cartridges and which do not require regular servicing—can be installed in large numbers. This innovative technology has become largely all pervasive in the retail trade and in public transport in particular.
In the recent past, however, increasing concerns have been raised about the environmental compatibility of certain color developers, and these, while not necessarily being science based, cannot simply be disregarded by the industry and particularly by commerce. Particularly heat-sensitive recording materials containing                bisphenol-A, i.e., 2,2-bis(4-hydroxyphenyl)propane, and        bisphenol-S, i.e., 4,4′-dihydroxydiphenyl sulphone,have thus attracted criticism, and this led primarily to an increased use of        Pergafast® 201, i.e., N-(p-toluenesulphonyl)-N′-3-(p-toluenesulphonyl-oxyphenyl)urea, from BASF Corporation, and        D8, i.e., 4-[(4-(1-methylethoxy)phenyl)sulphonyl]phenol.Having regard to this complete and abrupt change in the marketplace, the inventors recognized a growing and sustained need for further heat-sensitive recording materials that are accepted by the public as unconcerning in health and environmental terms and that stand out from the mass of possible heat-sensitive recording materials by the particular virtue of utilizing as color developers such components as are completely or very largely biodegradable and as are also in addition approved by the public as at least health-neutral.        